The cubes were billions of devices used by the Shakri in order to confuse, observe, and ultimately attack the Human race.
Properties
Each cube had its own unique function when they were activated for a total of forty-seven minutes after a dormant total time of a year.
They were extremely resilient. During tests, UNIT had cubes subjected to temperatures of plus and minus 200 Celsius, simulated a water depth of five miles, dropped one out of a helicopter at 10,000 feet and rolled their best tank over it.
History
The cubes were sent to Earth overnight from the Shakri Ship via seven transmats, and were investigated by UNIT to be found dormant and un-moving. The Doctor wanted them to be vulnerable, but nothing happened until the cubes activated after precisely one year. By this time, the cubes had become commonplace to the Human race, and could be found in almost every single house and place of work in the world.
After activating, each cube displayed unique characteristics. One opened and closed at the sides; one rattled; one took a reading of Amy Pond's pulse by stabbing at her palm with hidden spikes; one hovered, browsed the Internet and attacked the Doctor with a blaster hidden inside; one caused mood swings in the nearest human; one emitted fire for no discernable reason; and one played an endless loop of the "Chicken Dance" song.
After forty-seven minutes of this random activity, the cubes all simultaneously shut down, then started spontaneously counting down from seven. At the end of the countdown, each cube caused the heart of the nearest human to turn off; one also turned off the Doctor's left heart.
A second wave of cubes were commissioned to the Earth, but the Doctor changed their function to restart the hearts of the people who had been killed by the first wave. (TV: The Power of Three)